FIG. 2 shows a device for detecting ionizing radiation such as .alpha. rays reported in an Lecture No. 483 the National Conference of Electronics and Communication Society of 1986. In FIG. 2, reference numeral 1 designates a p type semiconductor substrate, reference numeral 2 designates an n-type region (n.sup.+ region) provided in the p type semiconductor substrate 1. Reference numeral 3 designates a precharge transistor for electrostatically precharging the n-type region 2 to a positive voltage with the gate voltage being controlled by a signal input to a terminal 31. Reference numeral 4 designates a detection buffer for outputting impedance converted voltage of representing the voltage variation detected at the n type region 2. The terminal 11 of the p-type substrate 1 is biased with a negative voltage with relative to the source voltage of the precharged transistor 3 or the positive power supply voltage 42 of the detection buffer.
This ionized radiation detector operates as follows:
At first, when .alpha. rays are incident to the n.sup.+ region 2 of the radiation detector, as shown by an arrow A, electron-hole pairs are generated and excited by the radiation energy on the incident region. When the n-type region 2 is charged with a positive voltage relative to the p-type substrate 1 by the precharge transistor 3, the p-n junction 12 is reverse biased, and the electrons of the excited electron-hole pairs towards the n-type region 2 to counteract the positive charges of that region, thereby lowering the voltage of the n type region 2. This lowering of the voltage is impedance converted by the detection buffer 4 and outputted to the output terminal 41. The n type region 2 is again precharged by the precharge transistor 3 to a positive voltage during a predetermined period to prepare provides for the next incidence of .alpha. rays.
In the above described operation, the number of generated electron-hole pairs varies depending on the rays angle of .alpha. incidence, and as a result, electron currents flowing through the n type region 2 also vary. In such a case, the radiation detector outputs a voltage in accordance with the direction of incidence. This voltage also varies dependent on the variation of the energy of the incident radiation, thereby rendering impossible to determine the direction of incident from the amplitude of the output voltage.
The use of the prior art ionizing radiation detector, which uses the above described detector to detect the direction of incidence, must determine whether the detector has directionality as part of its sensitivity. If the detector has directionality, the user must find the direction of incidence, or if the detector does not have directionality, the user must provide a collimator on the front surface of the detector. Furthermore, to obtain the direction of incidence it is necessary to determine the direction of the maximum intensity by conducting a number of times the measurement of putting the detector in various directions. Furthermore, it is impossible to use this method. When the intensity of the incident ionized radiation varies with the passage of time because the detected output will change due to intensity variation during the many measurements.